This invention pertains to dual flow passage conduit and more particularly to dual flow passage drill stem used for drilling holes in the earth.
An example of dual conduit drill stem is the construction disclosed in U.S. Pat. Nos.
2,657,016 -- Grable PA1 2,701,122 -- Grable. PA1 1,862,260 -- Edmunds PA1 1,954,454 -- McFarland PA1 2,850,264 -- Grable PA1 3,208,539 -- Henderson. PA1 3,273,660 -- Jackson et al PA1 1,868,400 -- Stover PA1 1,981,863 -- Harris.
For various reasons difficulty is experienced maintaining the integrity of such dual conduit drill stem when used in long drill strings where severe tensile and compressive loads are encountered, or in large holes when the pipe is laterally unsupported and excessive bending moments are experienced, or in air drilling where there may be temperature differentials due to hot air from the compressor or severe vibration due to high rates of flow of drilling fluid. Under certain conditions the connections between the tube and pipe tend to break loose. In other cases, welds between the tube and the tube centralizers or between the tube and tube connectors fail. Similar problems may be experienced with fluid transmission lines due to rapid temperature changes, e.g., from night to day in the desert or in the arctic, or when carrying hot liquids.
One attempt at solving the problem is to leave the tube unconnected to the pipe except at a single point. Examples of this type of construction are shown in U.S. Pat. Nos.
It has also heretofore been proposed to support the tube within the pipe of dual flow passage drill stem by means of elastomeric supports at one end of the inner tube as disclosed in the U.S. Pat. No.
And in U.S. Pat. application Ser. No. 643,254 -- Kellner et al
Filed Dec. 22, 1975, assigned to the same assignee as the present application. The elastomer supports in the Jackson patent and in the prior Kellner et al application are of the shear type wherein the elastomer disposed between inner and outer cylindrical surfaces coaxial with the tube, is placed in shear when the tube moves axially. In the case of the Jackson patent, heavy elastomer ribs between pipe and tube provide a releasable support to enable single flow passage drill stem to be converted to dual flow passage drill stem. In the case of the Kellner et al application, the conduit is intended primarily for horizontal boring. Also compare the construction of U.S. Pat. No.
wherein elastomer seals are compressed between adjacent tube ends.
Other forms of dual conduit drill stem are known in which the tube is provided with connectors having inwardly facing shoulders engaging outwardly facing shoulders in the tool joints forming the pipe connectors. See for example U.S. Pat. No.
In the Harris construction the tube is placed under slight axial tension by screwing the box tube connector onto the tube.
A construction in which the tube is releasably connected to the pipe at both ends and is preloaded in compression therebetween is shown in U.S. Pat. No. 3,471,177 -- Garrett et al.
In the pending U.S. Pat. application Ser. No. 535,193 -- Olson
filed Dec. 23, 1974, assigned to the same assignee as the present application, there is disclosed a construction which the tube is rigidly connected to each end of the pipe but the tube is parted intermediate its ends, the two parts being connected by a resilient coupling which serves to dampen vibrations as well as preventing undue load stress on the tube. In this regard it is to be noted that in all rotary drilling operations the drill stem is subjected to axial vibration. Such vibration may be induced by the bit as it repeatedly rotates past a particular high obstruction in the bottom of the hole, or by the teeth of rolling cutters successively impacting the bottom of the hole, or by impact imparted to the drill by a motor as in hammer drills and vibratory drills.
In connection with vibratory drilling of fairly large diameter, relatively shallow holes for posts for supporting the trans-Alaska pipe line, difficulty was experienced with dual conduit drill stem of the type employing a tube rigidly connected to the pipe at both ends. The welds holding the tube connectors to the tube failed. It was not known what was the cause of the problem, e.g. temperature differential expansion, or differential loading, fatigue failure, or resonant force build-up, or some other factor such as low temperature or a combination of several factors.